1. Field of the Invention
The invention relates to a spoke made of a composite material for a tension-spoke wheel, as well as to a wheel including such a spoke, especially a wheel whose rim and/or hub are also made of a composite material.
2. Description of Background and Other Information
Conventionally, a tension-spoke wheel, such as a bicycle wheel in particular, includes a peripheral rim, a central hub, and spokes connecting the rim to the hub. Generally, these various elements are entirely or partially made of metal or of a composite material.
Typically, such a composite material is constituted of fibers with high mechanical properties, especially carbon fibers or fibers of an equivalent material that are embedded in a resin matrix. Composite materials have a very high ratio between the modulus of elasticity (E) (tension) and density (d). This ratio yields the specific modulus of the material.
For carbon, E=115,000 MPa (MegaPascals) and d=1.55 for UD carbon fibers. By comparison,                for stainless steel, E=190,000 MPa and d=7.8;        for titanium, E=110,000 MPa and d=4.55;        for aluminum, E=71,000 MPa and d=2.8.        
Carbon, because of its high specific modulus, spokes can be made that are rigid and strong in tension but which have a reduced volume and a reduced weight. A spoke made of carbon fibers also has a high fatigue strength. Moreover, such spokes can easily be shaped in a mold and, in particular, can be given an aerodynamic profile.
It is also known to make a rim out of a composite material, starting with a core that is wrapped in layers of composite material, or with an inflatable bladder, or yet with two rim portions assembled to one another. Patent documents EP1231077 and U.S. Pat. No. 6,761,847 describe the construction of such a rim made of a composite material.
Patent documents WO00/35683 and U.S. Pat. No. 6,899,401 describe the construction of a wheel, the spokes of which are made of a composite material.
To fasten composite spokes to a rim, patent document EP1044827 proposes to bend the spoke ends over themselves on a core in order to form a buckle, the cross section of which is expanded by means of a central core. The ends are fastened, either directly by nesting the expanded portion in a housing of the hub, or via an auxiliary end piece forming a cap for fastening to the rim.
Patent document U.S. Pat. No. 6,036,279 discloses a construction of the same type. The spoke ends are flattened to form enlarged heads. One of the T-shaped heads is fastened to the hub by being embedded therein, the other being fixed to an intermediate cap.
These methods of construction yield good results; however, they require the use of auxiliary elements, especially metallic elements that weigh down the wheel.
Another solution involves making a rim, spokes and, if necessary, a hub body out of composite material, and assembling these various elements by gluing with resin that forms the matrix of the composite material, or with glue.
Patent documents WO93/19944 and U.S. Pat. No. 5,104,199 describe a construction of this type, in which the spokes are assembled directly to the rim and to a central flange. The wheel is thus made in two monolithic halves that are then assembled to one another. To tension the spokes and to form the hub body, the two central flanges are spaced apart, and a spacer is positioned to keep them at the predefined spacing.
The wheel built in this manner is light and rigid. Indeed, the lack of auxiliary fastening elements allows for a significant weight reduction.
However, fixing the spoke directly to the rim deprives the connection between these two elements of any degree of freedom that was previously available with a nested connection or when fastened via a cap. Moreover, the rim and the spoke are two completely different elements geometrically. A spoke made of a composite material is slightly flexible or not flexible, mainly in a plane in which it has its largest cross-sectional dimension. In any event, it is markedly less flexible than a metallic spoke. Thus, it has been noted that certain impacts applied to the wheel, in particular frontal impacts, could weaken the spoke in the area of its connection with the rim, and therefore could diminish its fatigue strength.
A similar problem arises in the area of the hub if, in the same fashion, the spoke made of a composite material is assembled to the hub via complete embedding, without any degree of freedom.